Cut the butternut squash, onions, and apples into 1-inch cubes. Place them on a sheet pan and toss them with the olive oil, 1 teaspoon salt, and 1/2 teaspoon pepper.
Divide the squash mixture between 2 sheet pans and spread in a single layer.
Roast for 35 to 45 minutes, tossing occasionally, until very tender.
Meanwhile, heat the chicken stock to a simmer. When the vegetables are done, put them through a food mill fitted with the medium blade. (Alternatively, you can place the roasted vegetables in batches in a food processor fitted with the steel blade. Add some of the chicken stock and coarsely puree.)
When all of the vegetables are processed, place them in a large pot and add enough chicken stock to make a thick soup. Add the curry powder, 1 teaspoon salt, and 1/2 teaspoon pepper.
Taste for seasonings to be sure there's enough salt and pepper to bring out the curry flavor. Reheat and serve hot with condiments either on the side or on top of each serving.